The Unit's continuing focus on the vocal behavior of nonhuman primates centered primarily on playback studies of naturally produced species calls. In this, our emphasis continues to be on affiliative calls which we have identified as being used in a 'conversational' manner. We now have evidence that squirrel monkeys differentiate chucks originating from within their own group, as opposed to those of neighboring groups, or strangers. Playback of recorded chucks from group members elicited responses similar in latency, type, and frequency to naturally-emitted chucks, while those from non-group members received only mild interest, or agonistic responses. Furthermore, monkeys distinguished between chucks uttered in the context of a "question" and those uttered as "answers'. The latter distinction was made only for chucks originating from within the group, indicating that such information is only available (or only used) for individuals who are familiar. Additional data showing that animals responded to the chuck calls of relatives (a group of mothers and daughters) out of contact for years suggests that "familiarity" may encompass some as yet unidentified common properties in the chuck calls of animals sharing genes or rearing environment. In addition, we continue to gather data on early development of vocal production and usage in squirrel monkeys. Extension of the observation period from 12 to 18 months confirms that some calls mature quite late, while others drop out of the repertoire. A small number of calls having structural properties similar to those of the chuck may provide evidence for a developmental continuum between infantile and adult calls.